1. Field of the Invention
The present invention relates to a Raman gain module for Raman-amplifying signal light by pumping light and an optical transmission system using the same.
2. Related Background Art
An optical fiber amplifier optically amplifies signal light transmitted through the optical fiber transmission path of an optical transmission system so as to compensate for a transmission loss in the optical transmission path. The optical fiber amplifier installed on the optical transmission path comprises an optical amplification optical fiber which also functions as an optical transmission path and a pumping light supply means for supplying pumping light to the optical amplification optical fiber. When signal light is input to the optical amplification optical fiber to which the pumping light is being supplied, the signal light is optically amplified by the optical amplification optical fiber and output.
For such an optical fiber amplification, a rare-earth-element-doped fiber amplifier in which a rare earth element such as Er (erbium) is doped and a Raman amplifier using a Raman amplification phenomenon by induced Raman scattering are used.
For the rare-earth-element-doped fiber amplifier (e.g., EDFA: Erbium-Doped Fiber Amplifier), an optical fiber (e.g., EDF; Erbium-Doped Fiber) in which a rare earth element is doped is used as an optical amplification optical fiber. This fiber amplifier is installed in, e.g., a relay station of the optical transmission system as a module. On the other hand, in the Raman amplifier, a silica-based optical fiber that constructs the optical fiber transmission path is used as a Raman amplification optical fiber.
The above-described Raman amplifier can be constituted as a distributed-type optical amplifier which inputs pumping light to an optical fiber for optical transmission together with signal light and compensates for a transmission loss by Raman amplification. In addition, like an EDFA, the Raman amplifier can be installed at a predetermined position in, e.g., a relay station on the optical transmission path as a module and used as a lumped-type optical amplifier module for optically amplifying input signal light by a predetermined net gain to obtain output signal light.
However, when the Raman amplifier is used as a lumped optical amplifier, since the Raman amplification optical fiber necessary for optical amplification is longer than an EDFA or the like, the influence of a nonlinear optical effect such as self phase modulation or four wave mixing becomes large. For this reason, the degradation in signal light transmission quality due to the nonlinear optical effect in the Raman amplification optical fiber becomes conspicuous.
The present invention has been made in consideration of the above problem, and has as its object to provide a lumped Raman gain module in which degradation in signal light transmission quality due to the nonlinear optical effect is suppressed, and an optical transmission system using the Raman gain module.
In order to achieve the above object, according to the present invention, there is provided (1) a Raman gain module for Raman-amplifying signal light in a predetermined amplification wavelength band by pumping light, characterized by comprising (2) a plurality of Raman amplification optical fibers connected in series to Raman-amplify the signal light in the amplification wavelength band and having different wavelength dispersion values, (3) one or a plurality of pumping light supply means for supplying the pumping light to the plurality of Raman amplification optical fibers, respectively, and (4) a housing for accommodating the plurality of Raman amplification optical fibers and the one or plurality of pumping light supply means.
In a lumped Raman amplifier formed using a single Raman amplification optical fiber, wavelength dispersion in the amplifier module cannot be controlled because of its arrangement. Hence, depending on the value of wavelength dispersion of the Raman amplification optical fiber, such an optical transmission condition may be generated that the dispersion value accumulated in signal light during transmission through the Raman amplification optical fiber becomes large or the signal light is transmitted through the Raman amplification optical fiber in an almost zero dispersion state.
When the dispersion value of the signal light becomes large, SPM (Self Phase Modulation) or GVD (Group Velocity Dispersion) occurs. When the signal light is transmitted in an almost zero dispersion state, XPM (Cross Phase Modulation) or FWM (Four Wave Mixing) occurs. When such a nonlinear optical effect occurs during transmission through the Raman amplification optical fiber, the signal light transmission quality degrades due to the influence.
To the contrary, in the above-described Raman gain module, a lumped Raman amplifier is formed using a plurality of Raman amplification optical fibers that are connected in series and have different wavelength dispersion values. With this arrangement, wavelength dispersion in the optical transmission path in the amplifier can be controlled, and accumulation of dispersion into signal light and transmission in an almost zero dispersion state can be reduced. Hence, a Raman gain module can be implemented in which degradation in signal light transmission quality in the amplifier due to the nonlinear optical effect is suppressed.
According to the present invention, there is also provided an optical transmission system characterized by comprising an optical transmission path formed using an optical fiber through which signal light is transmitted, wherein the above-described Raman gain module is installed in a relay station for relaying the signal light transmitted through the optical transmission path.
When a Raman gain module having the above arrangement is used as a lumped optical amplifier installed in a relay station of an optical transmission system, an optical transmission system which can suppress degradation in signal light transmission quality and reliably transmit the signal light from the transmission station to the reception station can be implemented.